This invention relates to methods and apparatus for filtering digital data signals; and more particularly, it relates to methods and apparatus for filtering images of the type which are made up of an array of such signals.
In the prior art, visual images have been converted into an array of digital data signals called pixels. Also in the prior art, these arrays of pixels have been filtered in various ways in order to enhance the image. However, an image usually includes huge numbers of pixels, and conventional image filtering processes require scores of multiplications and additions to be performed for each pixel. Consequently, conventional image filtering processes require a great deal of processing time and are very complicated.
An attempt to filter images in a simpler and quicker fashion is disclosed in a U.S. patent application Ser. No. 157,199 by Chabries and Christiansen which is entitled "Method of Forming Visual Images in Radar by Utilizing Preconvolved Quantized Vectors" filed Feb. 17, 1988, now U.S. Pat. No. 4,977,60. In that application, a method of filtering images by convolution is described in which conventional fast Fourier transform operations and convolution operations are eliminated. Instead, those operations, which are very time consuming, are replaced by a repetitive sequence of shifting and adding; and that enables the Chabries-Christiansen filtering to be performed relatively quickly.
However, a major limitation of the Chabries-Christiansen method is that it is limited to the convolutional type of filtering. This is evident from the fact that the convolution operation is linear, and so the principle of superposition applies. Superposition is what is occurring in the repetitive shift and add sequence. Thus, the Chabries-Christiansen method simply will not work for filtering which is nonlinear. Such nonlinear filtering occurs in many ways such as, for example, whenever one of the operations that is to be performed on the pixels involves a squaring or a square root.
Also, another limitation of the Chabries-Christiansen filter is that its multiple shift and add operations require a certain amount of circuitry to be carried out, and that adds to the filter cost. Further, performing multiple shift and add operations inherently takes a certain amount of time, which limits the maximum speed of the filter.
Accordingly, a primary object of the invention is to provide another method and apparatus for filtering digital data which completely eliminates all of the above limitations.